The present invention relates to a framing system for digital signals having a frame made up of an n-bit framing pattern and m-bit information, n+m bits long in total, and a clock period of T seconds. More particularly, the present invention is concerned with a framing system applicable to a situation wherein the probability is very low that a same pattern as an n-bit framing pattern appears in m-bit information, i.e. a framing system of the unique word detection type.
A unique word detection type framing system is often required for systems in which the clock of received digital signals undergoes substantial phase fluctuation, such as satellite communication systems and digital audio systems.
A predominant system for the unique word detection type framing is one which employs a window for detection. A window or time slot is defined which extends to opposite sides of the position of a framing pulse which presently indicates a correct frame position. Every time a clock pulse appears, an n-bit long or shorter digital signal arrived up to that time is observed and whether or not the resulting observed pattern is coincident with a framing pattern is determined. If the two patterns are coincident, the frame position is immediately regarded as a new frame position at the instant of coincidence detection. If no coincidence pulse is found in the window and such a state repeats itself predetermined N times, the system immediately enters a hunting state to search a new frame position. The number of times, N, is the parameter which is generally referred to as the "frequency of forward protection".
This kind of prior art method is effective only when the autocorrelation function of the framing pattern is so sharp that the appearance of a coincidence pulse at a position deviated from the present frame position due to a random error caused by noise and the like is least probable. This will be cleared up by the following analysis.
Suppose that an n-bit framing pattern is (a.sub.1, a.sub.2, . . . , a.sub.n) and this, in combination with information in the "k" and "k+1" frames, is being observed as a bit stream EQU . . . , b.sub.k,m-1, b.sub.k,m, a.sub.1, a.sub.2, . . . , a.sub.n, b.sub.k+1,1, b.sub.k+1,2, . . .
In this instance, the probability that an n-bit pattern, e.g., b.sub.k,m, a.sub.1, a.sub.2, . . . , a.sub.n-1, is incorrectly determined as the framing pattern a.sub.1, a.sub.2, . . . , a.sub.n is expressed as EQU P=p.sup.l (1-p).sup.n-l
where p is the bit error rate in transmission lines and l is the Hamming distance produced by ##EQU1## where .sym. indicates modulo 2 addition.
Supposing p=10.sup.-2, n=20 and l=5 by way of example, then P is on the order of not larger than 10.sup.-10 which is quite small compared to the stability of ordinary clock synchronization systems. In this case, therefore, the coincidence pulse appeared in the window may be regarded to indicate the frame position almost for certain.
However, in digital audio systems and the like, framing patterns with low code transition frequencies are often selected to lower the band of codes and thereby set up a lowest possible system operating frequency. Particularly, compact disc players belonging to digital audio systems are designed with n=22, a.sub.1 =a.sub.2 = . . . a.sub.11 =1 and a.sub.12 =a.sub.13 =. . . a.sub.22 =0. For these values, the Hamming distance l is EQU l=b.sub.k,m .sym.a.sub. 1+1
Then, if b.sub.k,m and a.sub.1 are common in sign, l=1 so that the probability P exceeds the stability of the clock synchronization system. It follows that the prior art framing system, which always regards a coincidence pulse appeared in the window as indicating a correct frame position and resets a frame counter each such time, cannot avoid noticeable deterioration to its characteristics due to the occurrence of random errors.
Further, while the window in the prior art method may be made larger in order to achieve stable framing against fluctuation in clock phase, it serves to proportionally reduces the forward protection function against random errors.
The present invention has been elaborated to overcome the drawbacks inherent in the prior art framing system as discussed hereinabove. In principle the present invention pays attension to the special characteristics of framing patterns and, when a coincidence pulse is found at a position different from the present frame position within a window, regards frame information as having been lost at that instant and causes the system into a hunting state to search a new frame position.